One of the conventional switching power supply devices is a current resonant converter including a series resonant circuit on the primary winding of a transformer. The current resonant converter includes the series resonant circuit including a resonant inductor and a resonant capacitor connected in series to a primary coil of the transformer, and a pair of switching elements to control current flowing from a DC-voltage input terminal to the series resonant circuit. The switching elements are alternately turned on or off at a frequency near the resonance frequency such that sinusoidal current flows in the primary coil of the transformer. The current induced in a secondary coil of the transformer is rectified with diodes and is smoothed with capacitors before output. The switching power supply device having such a configuration is known as a device exhibiting high power efficiency with low noise.
Unfortunately, such a current resonant converter including rectifying diodes in a secondary circuit may exhibit significantly decreased power efficiency due to large loss in the rectifying diodes. In order to reduce the loss in rectifying elements for improved power efficiency, for example, a device disclosed in Patent Literature 1 includes synchronous rectifying switching elements (MOSFETs) instead of the rectifying diodes in the secondary circuit and a secondary control circuit to detect terminal voltages of the secondary switching elements and turn on the secondary switching elements in synchronization with ON timings of the switching elements in the primary circuit.